1. Field of the Invention
This invention generally relates to radionuclide emission imaging and more specifically to improving quantitative information concerning physiological processes obtained by such imaging.
2. Description of Related Art
The use of radionuclide emission imaging for obtaining tomographic images is well known. These images visually present the distribution of a radionuclide tracer through a region of interest, typically in a human where regions of interest include the brain, heart, lungs, liver and other organs. This distribution of the tracer and associated compound models some physiological function such as blood flow or glucose metabolism in the region of interest.
Single photon emission computed tomographic (SPECT) imaging is one example of radionuclide emission imaging in which a collimator defines a plurality of photon flight paths to each of one or more photon detectors. The photon detectors receive photons emitted from the tracer in the patient along the various flight paths. Corresponding electronics analyze the received photons and assign to each flight path a vector having an intensity value that depends upon the number of photons received along that flight path. Processing these vectors by known reconstruction algorithms yields the final image.
In actual practice, SPECT and other radionuclide emission imaging systems must account for a number of other factors that can influence the accuracy of the final image and hence the information about the selected physiological process. This invention is directed to one such factor, namely the attenuation of photon energy as each photon travels from the emitting tracer nucleus through an object such as the patient's body to the collimator and detector. An accurate knowledge of this attenuation enables the intensity value associated with each vector to be compensated whereby the accuracy of the distribution can be translated into accurate absolute information, rather than just relative information.
One approach for determining attenuation involves using statistical models and assuming a substantially uniform density through the object or body being imaged. In some applications this approach provides reasonably accurate corrections. However, when the area being imaged includes significant density changes, as by organs of different densities and by organs and bones, this approach is not sufficiently accurate to provide accurate quantitative information.
U.S. Pat. No. 5,155,365 to Cann et al. for an Emission-Transmission Imaging System Using Single Energy and Dual Energy Transmission and Radionuclide Emission Data describes two other prior art approaches for obtaining photon attenuation emission. One involves the production of a computed tomography (CT) image using X-ray emissions independently of the emission image. Another involves the use of a radionuclide transmission source to obtain total length attenuation information. However, in each the attenuation measurement is made either before or after the emission image and in some situations at different locations. This complicates the correlation of the attenuation information with the emission image. The Cann et al. reference also discloses the use of a dual-energy X-ray source and a three-energy level detector system. The detector system discriminates among two discrete photon energy levels from the X-ray source and the energy level of photons emitted from the radionuclide tracer. This approach enables emission and attenuation to be obtained simultaneously and minimizes the problems correlating the attenuation and emission image data. However, the addition of a dual-energy X-ray apparatus effectively limits the application of this approach to single head SPECT systems. The apparatus is also more complex and expensive to manufacture.
U.S. Pat. No. 5,210,421 to Gullberg et al. for Simultaneous Transmission and Emission Converging Tomography discloses a three-head SPECT apparatus. In this patent, each head includes a photon detector system. A gantry supports the heads with equiangular spacing (e.g., 120.degree. for a three-head apparatus) and rotates those heads about the patient in order to provide improved sampling within a field of view that is a circle lying within and circumscribed by the heads. This apparatus obtains attenuation data by disposing a collimated radiation source diametrically across the field of view from one of the heads. The detectors in this head can discriminate between photons emitted from the radiation source and from the radionuclide tracer within the body. The flight paths for detected photons from the radiation source to the surface of the detector define a radiation fan that lies within an isosceles triangle in the image plane. The base of this triangle is located at the plane of the detectors and the apex of the triangle is located at the radiation source located midway between the other two detector heads. The physical construction of typical multiple head SPECT imaging apparatus limits the spread of the radiation fans (i.e., the angle at the apex of the triangle). Consequently the photons from the radiation source do not provide full and even sampling through objects that fill substantial portions of the field of view, as will be described. Moreover, reliance on the ability of the photon detectors to discriminate between photon energies to separate emission and transmission photons, complicates the apparatus and must be taken into account each time a different tracer is utilized.